1. Field of the Invention
The present invention relates to an electrostatically driven galvano-mirror used in e.g. an optical disk apparatus for controlling the direction of light emission.
2. Description of the Related Art
A typical electrostatically driven galvano-mirror is disclosed in e.g. "Silicon Torsional Scanning Mirror (IBM J. RES. and DEVELOP., Vol. 24, No. 5, September 1980)". As shown in FIGS. 12 and 13 of the accompanying drawings, the conventional galvano-mirror is provided with a lower substrate 100 and an upper substrate 102 bonded to the lower substrate 100. The upper substrate 102 includes a frame 104, a mirror element 106 (formed with a mirror surface 106a), and two torsion bars 108 connecting the mirror element 106 to the frame 104.
With such an arrangement, the mirror element 106 is deflected in a torsional movement about the torsion bars 108 upon application of external forces to the mirror element 106. The mirror element 106 has a bottom surface on which a pair of first electrodes 110a, 110b are formed. Correspondingly, the lower substrate 100 is provided with a pair of second electrodes 112a and 112b facing the first electrodes 110a and 110b, respectively. The lower substrate 100 is formed integrally with a ridge 100a contacting with the mirror element 106. The ridge 100a extends along the aligned axes of the torsion bars 108.
When voltage is applied across the first electrode 110a and the second electrode 112a, the mirror element 106 is rotated counterclockwise in FIG. 13 by electrostatic force. When voltage is applied across the other first electrode 110b and the other second electrode 112b, the mirror element 106 is rotated clockwise. Such electrostatic force is proportional to the area of the respective electrodes. Thus, for actuating the mirror element 106 with a low voltage, the area of the first electrodes 110a, 110b needs to be large, which may cause the electrodes 110a, 110b to cover almost the entirety of the lower surface of the mirror element 106. The size of the second electrodes 112a, 112b is determined in correspondence to the size of the first electrodes 110a, 110b.
In the conventional galvano-mirror described above, the mirror element 106 in motion tends to be subject to unfavorable damping due to the viscosity of the air present between the mirror element 106 and the lower substrate 100. Consequently, it is difficult to properly control the movement of the mirror element 106.
For reducing such viscous air-damping, the lower substrate 100 may be formed with a plurality of grooves facing the mirror element 106, as taught in JP-A-9(1997)-146034 for example. However, the additional processing of such grooves may make the fabrication procedures of the galvano-mirror disadvantageously complex. As a result, the production efficiency is lowered, while the cost is unduly increased.
Another problem of the conventional galvano-mirror of FIGS. 12 and 13 is that the mirror element 106 may be displaced sideways upon application of voltage across the first electrode 110a and the second electrode 112a (or across the other first electrode 110b and the other second electrode 112b). The mirror element 106 is moved in this manner since the electrostatic force generated by the voltage application has a horizontal component acting on the mirror element 106. Such sideways displacement may render the posture of the mirror element 106 unpredictable. Thus, desired control accuracy in operating the mirror element 106 may be unobtainable.
The above-described sideways displacement of the mirror element 106 may be reduced by attaching the mirror element 106 to the ridge 100a in a deflectable manner, as taught by JP-A-5(1993)-119280. Specifically, the bottom surface of the mirror element 106 may be formed with a groove into which the top of the ridge 100a is received. The drawback of this arrangement is that the production cost tends to be increased because it is necessary to precisely form the groove in the mirror element 106 at the right position. The ridge 100a also needs to be formed and positioned accurately.